This invention relates to luminescent phosphor compositions useful in preparing X-ray screens, cathode ray tubes and the like, and particularly to high efficiency luminescent phosphor compositions having reduced lag.
Luminescent phosphor compositions useful, for example, in preparing radiographic X-ray screens have been described extensively in the prior art. Recently, a number of so-called "high efficiency" phosphors have been developed which are activated by rare earths. These rare earth-activated phosphors suffer from various deficiencies, one of which is delayed fluorescence or "lag". This prior art is broadly discussed by Stevels in Medicamundi, Vol. 20, #1, 1975 pp. 12-22. Europium-activated fluorohalides (e.g. BaFCl:Eu) are included within this group of high efficiency phosphors. Lag is a particularly common deficiency among the europium-activated fluorohalides.
Lag is continued screen emission after excitation with X-rays has been discontinued. In roentgenology uses, excess lag cannot be tolerated since exposed screens used in this application can cause exposure of fresh photographic films. This is a particular problem in the modern X-ray facility where high speed machines are used which automatically place fresh film in contact with the X-ray screen immediately following exposure. Thus, the fresh film can be exposed even when contacted with a screen made from phosphors having relatively short lag. Of course, screens made from phosphors which exhibit longer lag cannot be tolerated even with manual changing of the film.
Various methods to surmount the problem of lag have been tried. One method has been to mix additives with the phosphor composition but these tend to discolor the phosphor or reduce its over-all output. Another method, in the case of rare earth activation, is to add larger amounts of the rare earth to the composition to reduce the lag. Rare earths are, however, expensive so that this solution to the problem is uneconomical.